ICS Advent PCI-ICOM485/4, Руководство по эксплуатации продукта

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Manual Number: 00650-148-1 Page 7
Chapter 2: Functional Description
The PCI-ICOM485/4 (/2) Serial Interface Card was designed for effective multipoint transmission
on long communication lines in noisy environments using RS-485 differential line drivers and RS-485
(EIA485) protocol. The data lines are opto-isolated from the computer and from each other to
assure communication when large common mode noise are superimposed. The card is 12.283
inches long and may be installed in 5-Volt PCI-bus slots of IBM PC or compatible computers. The
card features two or four (or two) independent, asynchronous RS-485 serial ports, type 16550
buffered UARTS, and, for Windows compatibility, automatic control to transparently enable/disable
the transmission drivers. The output stages are optically coupled to provide electrical isolation be-
tween the computer and the I/O.
RS-485 Balanced Mode Operation
The PCI-ICOM485/4 (/2) supports RS-485 communications and uses differential balanced drivers
for long range and noise immunity. RS-485 operation involves switchable transceivers and the
ability to support multiple devices on a single “party line”. The RS-485 specification defines a
maximum of 32 devices on a single line. The number of devices served on a single line can be
expanded by use of “repeaters”.
The PCI-ICOM485/4 (/2) also has the capability to add load resistors to terminate the communica-
tions lines. RS-485 communications require that one transmitter supply a bias voltage to ensure a
known “zero” state when all transmitters are off. Also, receiver inputs at each end of the network
should be terminated to eliminate “ringing”. The PCI-ICOM485/4 (/2) supports biasing by default
and supports termination by jumpers on the card. If your application requires the transmitter to be
un-biased, please contact the factory.
Comm Port Compatibility
Type 16550 UART’s are used as the Asynchronous Communication Element (ACE). These in-
clude a 16-byte transmit/receive buffer to protect against lost data in multitasking operating sys-
tems, while maintaining 100% compatibility with the original IBM serial port. The system assigns
the address(es).
A crystal oscillator is located on the card. This oscillator permits precise selection of baud rates up
to 115,200 or, by changing a jumper, up to 460,800 with the standard crystal oscillator.
The driver/receiver used, the SN75176B, is capable of driving extremely long communication
lines at high baud rates. It can drive up to ±60 mA on balanced lines and receive inputs as low as
200 mV differential signal superimposed on common mode noise of +12 V or -7 V. In case of
communication conflict, the driver/receivers feature thermal shutdown.
Communication Mode
The PCI-ICOM485/4 (/2) supports half-duplex communications with a two-wire cable connection.
Half-duplex allows traffic to travel in both directions, but only one way at a time. RS-485 commu-
nications commonly use the half-duplex mode since they share only a single pair of wires.